With input/output consolidation achieved using technologies such as Fiber Channel over Ethernet (FCoE), Ethernet and Fiber Channel (FC) traffic can use the same links and port-channels between devices. A port channel is an enhancement to Layer 2 Ethernet networks (now standardized as IEEE 802.3ad PortChannel technology), in which multiple links between two participating devices can use all the links between the devices to forward traffic through the use of a load-balancing algorithm that balances traffic across the available Inter-Switch Links (ISLs) while also managing the loop problem by bundling the links as one logical link.
One limitation of the port channel technology is that it operates only between two devices. In large networks, the support of multiple devices together is often a design requirement to provide for an alternate path. This alternate path is often connected in a way that would cause a loop, limiting the benefits gained with PortChannel technology to a single path. To address this limitation, a technology called virtual PortChannel (vPC) was developed. When configured to implement virtual PortChannel technology, each switch in a pair acts as a vPC peer endpoint, that is, like a single logical entity to PortChannel-attached devices, but the two devices that act as the logical PortChannel endpoint are still two separate devices.
When two types of traffic, e.g., Ethernet traffic and FCoE traffic, are mixed over a virtual PortChannel, problems are created that can result in poor load-balancing of traffic over certain virtual PortChannel links.